Device for cutting threads in the walls of the acetabular cavity in humans

ABSTRACT

The invention relates to total hip joint replacements in humans in which the ball-like head of the femur and the cup-shaped cavity of the acetabulum are replaced by metal, plastic or ceramic substitutes and, more particularly, to a device for cutting screw threads in the walls of the acetabulum to receive a threaded substitute hip socket.

BACKGROUND

Since the late 1960's it has been customary to utilize a plastic cementto fixedly secure the substitute hip components employed in total hipjoint replacement surgery. It has been found, however, that this methodof fixing the components into the patient's surrounding bone has seriousdrawbacks because the cement often becomes brittle and cracks afterbeing in place a few years. As a result the substitute components loosenand migrate from their proper positions thereby causing both pain anddeformity and necessitating further corrective surgery.

To avoid these problems it is now preferred to utilize other methods ofattaching the substitute hip components to the patient. A frequentlyused method for fixing the acetabular component into the hip bone socketcavity, the acetabulum, is to form screw threads on the outer surface ofthe acetabular component so that it can be screwed directly into thepreviously prepared acetabulum. The exterior shape of the substituteacetabular components currently available vary considerably, butgenerally are hemispherical, truncated cones or cylinders, with screwthreads of various size, shape and pitch formed on their outer surfaces.Preparation of the acetabulum is accomplished by chiseling, reaming orscraping the interior of the roughly hemispherical hip socket cavity toremove cartilage and some or all of the bony wall until the acetabulumis modified to substantially conform to the size and shape of theacetabular component that will be fitted into it. While some acetabularcomponents have self tapping threads on their exterior surfaces topermit the component to be screwed directly into the preparedacetabulum, other components require that the bony wall be threaded witha tapping device before the acetabular component can be inserted intothe hip socket cavity.

As presently done, this threading operation is carried out using a handtap or plug having the same size and shape as the prepared acetabulumwith cutting threads affixed to the plug's external surface. The use ofa conical hand tap requires that the threads be cut simultaneouslyaround the wall of the acetabulum, a very difficult task that requires agreat deal of physical force. Indeed, despite great force, it isdifficult to cut full depth threads into the relatively hard corticallayer of bone which lines most of the acetabulum. The force of turningthe conical tap into the bony wall can cause expansive strains on thewalls of the acetabulum causing it to fracture and prevent mechanicalfixation of the component. In addition, if the threads are not of fulldepth and of the proper size and shape, the threaded acetabularcomponent will not seat in the socket properly or deeply enough.Accordingly, it is customary in preparing the acetabulum to cut awaymost of this hard cortical outer layer of bone in the chiseling andreaming operation to expose the much softer and spongy cancellous bonebeneath it. Although it is much easier to create screw threads in thissofter cancellous bone it does not provide as strong a supporting mediumfor anchoring the acetabular component as the hard cortical layer andthe risk that the component will loosen in time is much greater.

The objective of the present invention is to provide a device forcutting threads in the acetabulum that does not have the drawbacks anddisadvantages discussed above. Another object of the invention is toprovide a tool that will enable threads to be easily cut in the hardcortical layer of the acetabular cavity.

Still another object of the invention is to provide a tool whose cuttinghead can be readily modified to match the contours of a bony hip socketthat has been reamed and shaped to conform to the size and shape of asubstitute acetabular component.

In the accompanying drawings, in which similar reference numerals referto similar parts,

FIG. 1 is a sectional view showing the placement of the substitutecomponents in a total hip joint replacement,

FIG. 2 is a sectional view showing the head of applicant's devicecutting threads in the hard, bony cortical wall of the acetabulum,

FIG. 3 is a perspective view of applicant's tool,

FIG. 4 is a sectional view through the head of applicant's tool, takenalong the line 4--4 in FIG. 3, and

FIG. 5 is an exploded view of applicant's tool.

In the course of a total hip joint replacement, the ball-like head onthe femur is replaced by a substitute metal, plastic or ceramic head.Because the bony hip socket or acetabulum is often diseased or deformed,the socket is also replaced by an acetabular component composed of thesame or complimentary materials or a combination of them. A presentlyavailable set of such substitutes are shown in FIG. 1. As will be seenthere, the substitute femoral head 10 and acetabular cup 12, with ahead-matching hemispherical socket 13, are formed from a high strength,super-hard ceramic while the substitute femoral stem 14 is formed from acobalt-chromium-molybdenum cast alloy. The stem of the femoral head 14is inserted into the medullary canal 16 of the femur 18 and locked therein accord with presently known procedures that do not concern us sincethe present invention relates to the placement of the acetabular cup.While the specific materials used in the substitute components shown inthe drawing have been identified, it is to be understood that there isnothing critical about them and that other constituent materials couldbe used just as well. In addition, the materials used could be coatedwith a porous coating commonly used to assist in the fixation of thecomponent to the bone since bone cells will grow into the pores of thecoating.

The acetabular cavity that forms the socket for a normal, healthy hipjoint is roughly hemispherical but not perfectly so. Furthermore, theroughly hemispherical shape can be greatly modified by diseases of thehip joint. For the most part, the walls of the acetabular cavity areformed by a hard, bony shell of varying thickness called the corticallayer 20. This fairly thin layer of hard, bony shell is supported fromits under surface by the softer, spongy cancellous bone 22 of thepelvis. Unlike the prior art procedures, applicant's device requireslittle preparation of the acetabulum. The superficial cartilage surfaceof the acetabululm is removed, but depending on the exact externalconfiguration of the acetabular component to be inserted, it is notnecessary to remove the hard, bony cortical layer 20 of the hip socket'swalls or bottom.

The applicant's thread cutting device comprises a hand tool having acutting head 24 shaped to closely match the configuration of theacetabular component to be inserted. Since the cutting head is matchedto the acetabular component it is important that the acetabularcomponent be of a shape that closely fits the particular shape of thepatient's acetabulum so that the cutting head will, in turn, closelymatch the size and shape of the hip socket cavity being replaced. Whenthis relationship is maintained, the external shape of the cutting head24 will closely conform to the patient's acetabulum and serve to keepthe tool centered in the acetabulum while the threads are being cut inthe walls of the socket and the threads will be of a uniform depth andshape. If the relationship is not maintained there will be a poor fitbetween the tool and the acetabulum and the tool will be able to wobbleor otherwise wander during the cutting of the threads with the resultthat the threads will have non-uniform shape and depth.

A central shaft 26 is enclosed within an independently rotatable driveshaft 28 for the cutting head, and a pair of handles 30,32 control theoperation of the device. As is best seen in FIG. 5, the central shaft 26extends throughout the length of the device. At its forward end thecentral shaft 26 is fixed to a generally cylindrical alignment disk 34that carries two axially disposed, parallel pegs 36,36 adapted to fitwithin two similarly sized locating holes 38,38 drilled in the bottom ofthe patient's acetabulum. Immediately behind the locating pegs 36,36 thealignment disk 34 has an arcuate flange 40 shaped to the inner curvatureof the cutting head 24. The function of this flange 40 is to positionthe alignment disk 34 in sliding engagement with the cutting head 24 soas to position and maintain the head at the forward end of the centralshaft 26 while still permitting rotation of the head relative to theshaft.

For the reasons previously described, the cutting head 24 hassubstantially the same shape as the threaded acetabular cup that will bepermanently secured in the patient's hip socket. In general, this shapewill be nearly hemispherical, although cutting heads in a number ofdifferent shapes and sizes are necessary in order to accommodate a fullrange of variations in patients' hip socket cavities. At its forwardend, the cutting head has a circular opening 42 to accommodate thealignment disk 34 and the side walls 44 of the opening 42 in the headare sized so that the head fits closely about the disk 34 and is alignedtherewith. The head 24 is also formed with an elongated, axial slot 46through which the cutting tool bit 48 protrudes as will be described inmore detail hereafter.

A tool carrier 50, with the cutting tool bit 48 fixedly mounted therein,is adapted for connection with the drive shaft 28 in a manner that willcause the cutting tool bit to rotate with the shaft yet be radiallyslidable with respect thereto. To this end the tool carrier 50 is formedlike the letter "C", that is, in a generally annular configuration butone that is not a complete circle. The inner surfaces 52,52 of thecentral opening 54 in the tool carrier 50 are straight and parallel andcorrespondingly shaped slots 56,56 are formed in the forward end of thedrive shaft 28 to permit the carrier to move outwardly of the shaft. Acoil spring 58 between the carrier 50 and the shaft 28 continuouslybiases the tool carrier toward the inner surface of the cutting head sothat the cutting tool bit 48 is always positioned to cut to the properdepth in the side wall while following the profile of the cavity'swalls. In the assembled position of applicant's tool shown in FIG. 4,the inner surfaces 52,52 of the central opening 54 in the tool carrier50 also straddle a drive block 60 that is fixedly mounted by screws62,62 to the inner surface of the cutting head 24. This arrangementoperates to transfer the rotational motion of the drive shaft intorotation of the cutting head and cutting tool bit. The drive block 60also serves to hold the alignment disk 34 locked into position in thecircular opening 42 of the cutting head 24. An end plate 64 closes offthe outer end of the cutting head 24 and maintains the drive shaft 28 init's central location.

The outer drive shaft 28 surrounds the inner central shaft 26 and isrotatable with respect to it. Rotation of the drive shaft 28 occurs whenthe double handle 30 at the end of the tool is rotated. A lead screw 66is formed intermediate the ends of the central shaft 26 and a pair ofoppositely disposed thread followers 68,68 (only one of which is shown)are carried by the drive shaft 28. Rotation of the drive shaft relativeto the central shaft accordingly causes the drive shaft to slidelongitudinally along the central shaft. This helical rotation of thedrive shaft causes the cutting tool bit 48 in the head 24 to rotatehelically and cut threads into the hard, bony shell of the acetabularcavity. A knob 70 is provided on one end of the double handle 30 andthis knob is preferably lined up with the cutting tool 48 so that theposition of the knob 70 indicates to the surgeon exactly where thecutting tool is working. While the handle 30 operating the drive shaftis being rotated, the handle 32 on the central shaft 26 is held steadyso that the tool, having been oriented by the two location holes in thebottom of the acetabulum, is maintained in that orientation. The singlehandle 32 on this shaft is preferably lined up with the two locatingpegs 36,36 so that the surgeon may easily keep track of the position ofthese locating means at all times.

In the utilization of the applicant's tool, the acetabulum is preparedas previously described. Two location holes 38,38 are first drilled inthe bottom of the cavity. The handle 70 is rotated in a counterclockwisedirection to bring the cutting tool bit 48 to its outermost position.The surgeon then places the head of the tool into the patient's cavityand inserts the locating pins 36,36 in the holes 38,38. An assistantsurgeon maintains a steady pressure on the handle 32 of the tool tomaintain the pins in the holes and the tool in a fixed attitude. Sincethe cutting head 24 of the tool closely fits the patient's acetabularcavity the insertion of the tool into the cavity puts the cutting toolbit 48 in a position to start cutting a thread in the wall of thecavity. To this end the surgeon rotates the handle 30 which rotates thedrive shaft 28, cutting tool bit 48 and cutting head 24. As the driveshaft and cutting tool rotate the cutting tool bit 48 is also advancedand moves longitudinally in the slot 46. At the same time the spring 58keeps the tool cavity 50 outwardly displaced against the interiorsurface of the cutter head 24 on either side of the slot 46 so that thecutting tool bit 48 follows the contours of the prepared acetabularcavity while the threads are being cut. Rotation of the cutting head 24occurs around the alignment disk 34 on the end of the central shaft 26.In this way, the surgeon cuts a helical screw thread in the corticalwall of the acetabular cavity. Since only one thread is being cut at atime, the physical force required for the task is not excessive.

I claim:
 1. A device for forming threads in the wall of the acetabularcavity, said device comprising:(a) a cutting head shaped to closely fitwithin the acetabular cavity, (b) means for positioning said cuttinghead relative to the cavity, (c) a single cutting tool mounted toproject exteriorly of said cutting head, and (d) means for driving thecutting tool in a spiral path so as to cut threads in the generallyhemispherical wall of the acetabular cavity.
 2. A thread-forming deviceas set forth in claim 1 wherein the cutting head is carried by thepositioning means and rotatable relative thereto.
 3. A thread-cuttingdevice as set forth in claim 2 wherein the positioning means are mountedon a central shaft and the driving means for the cutting tool comprisesa drive shaft surrounding the central shaft and rotatable relativethereto.
 4. A thread-cutting device as set forth in claim 3 wherein thecentral shaft is provided with a lead screw and the the drive shaft withscrew thread followers whereby relative rotation of the drive shaft andthe central shaft moves the cutting tool through a spiral path.
 5. Athread-cutting device as set forth in claim 4 wherein the central shaftand drive shaft are each provided with manual means for holding thecentral shaft aligned relative to said cavity and for rotating the driveshaft.
 6. A device for cutting threads in the wall of the acetabularcavity, said device comprising:(a) a central shaft having a screw threadformed therein, (b) an alignment disk on one end of the central shafthaving means for preventing rotation of the central shaft, (c) a cuttinghead rotatably mounted on said central shaft and having a longitudinalslot formed therein, (d) a cutting tool mounted in and extendingexteriorly of the slot in said cutting head, (e) a drive shaft for thecutting tool and cutting head rotatable relative to the central shaftand having thread followers for engagement with the screw thread on thecentral shaft, and (f) handle means for rotating the drive shaftrelative to the central shaft to rotate the cutting tool in a spiralpath.
 7. A device for cutting threads in the wall of the acetabularcavity, said device comprising:(a) a rotatable cutting head shaped tosubstantially conform to the shape and size of the acetabular cavity andhaving a longitudinal opening therein, (b) a cutting tool located withinthe cutting head and having a cutting tool bit extending exteriorly ofsaid cutting head through the opening, (c) means resiliently biasing thecutting tool bit outwardly of said cutting head, and (d) means fordriving the cutting tool bit in a spiral path so as to cut threads inthe generally hemispherical wall of the acetabular cavity.